This invention relates to a system and process for recovery wherein steam and other hot gases are produced downhole in a gas generator located at the bottom of a borehole.
For the recovery of highly viscous oil from oil reservoirs, it has been found that hot water and steam piped downhole have been effective in reducing the viscosity of the oil so that it will flow and can be pumped to the surface. One of the problems encountered in piping steam downhole has been associated with heating and expansion of the well bore casing which often results in severe damage to the casing. Another problem arises from loss of heat through the casing from steam enroute to the bottom of the well. Moreover, the known systems cannot pump steam downhole or generate steam downhole at a depth below about 3,500 feet.
It is an object of the present invention to provide a system and process for generating steam and hot gases downhole, for recovery purposes, at a depth down to and below 3,500 feet.
It is another object of the present invention to provide a system and process by which steam and other hot gases may be produced by the combination and burning of a fuel and an oxidizer in a vented pressure vessel, known as a gas generator, located at the bottom of a borehole, thus avoiding the problems caused by heating the well casing and by loss of heat through the casing when hot water and steam are piped downhole. The gas generator comprises a housing forming a chamber which defines a combustion zone. The housing has an upper inlet end for receiving fuel and an oxidizing fluid and a restricted lower outlet for the passage of heated gases. An igniter is provided for igniting combustible gases in the combustion zone. Also provided is a cooling fluid annulus surrounding the chamber and having passages leading to the chamber.
It is a further object of the present invention to supply hydrogen and oxygen downhole to the gas generator for the formation of a combustible mixture which is ignited and burned in the combustion zone. The combustible mixture may be a stoichiometric mixture of hydrogen and oxygen or it may be hydrogen-rich. The gas generator and the combination products are cooled by introducing hydrogen into the cooling annulus or water supplied downhole to the gas generator. The hydrogen exhausted into the reservoir either by the burning of a hydrogen-rich mixture or by the hydrogen supplied to the cooling annulus, contains heat which is transferred to the oil to reduce its viscosity. Because of low molecular weight and high diffusivity, the hydrogen has the added advantage of being able to more readily penetrate the bed containing the oil and can therefore heat a larger bed volume more rapidly than can other gases. In addition, with certain bed compositions which may act as catalysts, the hydrogen can enter into a process normally referred to as hydrogenation to form less viscous hydrocarbons, thus reducing oil viscosity, both by heating and by combining with the oil.
For positive control of the flow of hydrogen and oxygen and to prevent the gas generator from being prematurely flooded, in the event that water is applied to the cooling annulus, remotely controlled valves are provided downhole near the gas generator. These valves are controlled from the suface for controlling the flow of hydrogen and oxygen to the gas generator and for controlling the flow of water to the cooling annulus, if water is employed for cooling purposes.
In the embodiment where water is employed for cooling purposes, water is supplied downhole through the borehole casing and hydrogen and oxygen are supplied through separate conduits extending through the borehole. In the embodiment wherein hydrogen is supplied to the cooling annulus, hydrogen may be supplied downhole through the borehole casing and oxygen is supplied through a separate conduit extending through the borehole.
The well casing is sealed just above the generator by an inflatable packer surrounding housing structure above the gas generator. In the embodiment employing water for cooling purposes, the packer is inflated by the hydrogen, whereby sealing is effected initially from the hydrogen pressure and finally from the pressure exerted by the water column in the casing. In the embodiment wherein hydrogen is supplied to the cooling annulus, of the gas generator, the packer may be inflated by the pressure of the oxygen, whereby sealing is effected initially by the oxygen pressure and then from the hydrogen pressure supplied through the well casing.
The remotely controlled valves, in one embodiment, are solenoid valves located downhole and controlled from the surface. In another embodiment, a single spool valve having separate valve passages in a valve spool is employed downhole and which is controlled remotely from the surface by a separate solenoid or by the hydrogen pressure.
Hydrogen is supplied from the surface by way of a hydrogen supply, a hydrogen metering valve, and a hydrogen flow meter, all of which are located at the surface. The oxygen is supplied from the surface by way of an oxygen supply, an oxygen metering valve, and an oxygen flow meter which also are located at the surface. In one embodiment, the desired hydrogen-oxygen ratio is maintained by the use of a hydrogen flow control located at the surface and which is slaved to a thermocouple supported by the gas generator. The hydrogen flow control outlet is coupled to the hydrogen metering valve for controlling the desired amount of hydrogen flow therethrough.
In order to reduce the number of conduits and electrical leads extending from the surface through the borehole, to the gas generator, a DC power igniter control may be located downhole to control ignition of the combustible mixture in the gas generator. The igniter control is actuated by a switch supported by the valve spool valve which is remotely controlled by the hydrogen pressure. In this embodiment, the desired hydrogen/oxygen ratio is maintained by a hydrogen-oxygen flow control coupled to the hydrogen metering valve and hydrogen flow meter and coupled to the oxygen metering valve and oxygen flow meter.
Although the preferred embodiment employs a fuel-oxidizer-cooling fluid combination of hydrogen and oxygen or hydrogen, oxygen, and water, provision is made for employing other fuel-oxidizer-cooling fluid combinations.